1. Field of the Invention
The invention relates to a method for the hydrolysis of starchy materials based on the use of amylase extracted from sweetpotato root outer tissues.
2. Description of Prior Art
Sweetpotato, a high starch content crop, is considered as a potential agricultural source of maltodextrin, syrup, and glucose conversion. Recent interest in energy conservation and in renewable energy sources has focused considerable attention to starch hydrolysis in glucose, maltose, and dextrins by microbial amylases. Many exciting changes happened within the food industry as a result of improvements and innovations in the use of these enzymes. Although many more unique applications are being found, the cost of manufacturing and subsequently the cost-effectiveness of enzymes and the recent ecological worry presented a barrier.
However, in addition to high starch, sweetpotato roots have a high content of amylases. The native amylases have important and well-documented influences on sweetpotato root storage and processing (Walter, W. M., Jr. et al., 1975, J. Food Sci., 40: 793-796). It is known that in baked sweetpotato roots, endogenous amylases hydrolyze part of the starch into maltose and longer chain polysaccharides, resulting in a sweet taste. Nevertheless, baking temperature is sufficient enough to rapidly inactivate plant amylases, and limits the starch hydrolysis.
Raw sweetpotato starch is less susceptible to sweetpotato amylase digestion than boiled starch granules (Hagenimana, V. et al., 1992, J. Agric. Food Chem., 40: 1777-1783), and sweetpotato root mashes held a long time at temperatures from 30.degree. to 80.degree. C. do not gain in soluble solids or sugars but their starch retrograded suggesting the insufficient reduction in chain length of starch components. The utilization of endogenous amylases in sweetpotato processing should involve heating roots or their puree above 60.degree. C. to gelatinize the starch fraction and to facilitate amylase action.
In starch hydrolysis materials such as maltodextrin, syrups, glucose, etc. from a granular starch feedstock, it is conventional practice to first subject an aqueous slurry of said granular starch material to a cooking or pasting operation, which is typically conducted at a temperature of from about 60.degree. C. to about 180.degree. C., and to thereafter convert the resulting cooked or pasted starch material to the desired starch hydrolysate product (e.g. typically maltodextrin, syrup or glucose) via one or more acid and/or enzyme hydrolysis operations.
In the case of enzyme-based hydrolysis operations, the cooked or pasted starch is typically treated for a relatively short period of time (e.g., 1 to 4 hours) under mildly acidic conditions (e.g., generally at a pH of about 6.0) and at a relatively high temperature (e.g., up to about 100.degree. C.) with bacterial thermostable .alpha.-amylase enzyme in order to convert said starch slurry to maltodextrin. Such treatment is generally referred to in the art as "thinning" reaction. In the event that the maltodextrin thus formed is the desired end-product, the crude hydrolysis reaction product is purified or used as such. On the other hand, if the desired ultimate endproduct is syrup, the aforementioned maltodextrin material is subjected to further hydrolysis to convert said maltodextrin material to the desired syrup product. This latter hydrolysis operation is commonly referred to in the art as a "saccharification" process or operation. When said saccharification operation is performed via acid hydrolysis techniques, it is typically conducted using a strong mineral acid such as hydrochloric or sulfuric acid; at a pH in the range of from about 1 to about 2; at a temperature in the range of from about 90.degree. C. to about 180.degree. C.; and for a time period or reaction time of from about 0.1 to about 2 hours.
The use of endogenous sweetpotato amylases in hydrolysis of starch requires two conditions: first, starch gelatinization prior to hydrolysis and secondly, preservation of endogenous amylase activities during gelatinization.
.alpha.-amylase plays a major role in the digestion of starch and the combined action of .alpha.- and .beta.-amylases is more effective than the action of .alpha.- or .beta.-amylase alone (Maeda, I. et al., 1978, Agric. Biol. Chem., 42: 259-267). The ubiquitous distribution of .beta.-amylases throughout the root and the localization of .alpha.-amylase in the outer layers of the root was recently reported (Hagenimana, V. et al., 1992, J. Agric. Food Chem., 40: 1777-1783).
It would be highly desirable to optimize the use of the endogenous amylases during processing of starchy materials. To do so, it was hypothesized that the outer tissue extract of sweetpotato containing both .alpha.- and .beta.-amylases would be used to hydrolyze starch without adding exogenous amylases.